From: "Saved by Internet Explorer 11" Subject: PAiA: How MS Stereo Works Date: Thu, 6 Mar 2014 15:38:53 -0800 MIME-Version: 1.0 Content-Type: multipart/related; type="text/html"; boundary="----=_NextPart_000_00CF_01CF3952.2DA4A4A0" X-MimeOLE: Produced By Microsoft MimeOLE V6.1.7601.17609 This is a multi-part message in MIME format. ------=_NextPart_000_00CF_01CF3952.2DA4A4A0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: file://C:\Users\R&D 1\Documents\Research\Microphones\Circuits\M-S Mic Technique\PAiA How MS Stereo Works.htm
=20Usually the most intuitive way to perform a task is the best ... but = not=20 always.
Since we have Left and Right ears, the most =
intuitive way=20
to encode stereo sound information is as Left and Right channels. And =
stereo=20
delivery media such as CD, tape, vinyl and older film do it just this =
way.=20
Similarly, when placing microphones to record a stereo signal the =
most=20
intuitive way to go about it is what's known as A-B mic'ing. Two mics =
are placed=20
some distance apart and pointed in the direction of the source to be =
recorded.=20
The mic on the left is recorded as the L channel and the one on right as =
R. On=20
the face of it this looks pretty fool-proof and when you listen to a =
ping pong=20
game recorded this way, sure enough - it sounds like stereo.
=
But when=20
the novelty of ping pong wears off and you progress to more sonically =
complex=20
sound sources (like musical instruments) some subtle problems begin =
showing up=20
that have to do with the fact that the mics are encoding the L and R =
information=20
not only as level differences but also as time differences. In the =
figure, the=20
distance from the "A" source to the "L" mic is greater than to the "R" =
mic so=20
the "L" signal is slightly delayed relative to the "R" signal.
On playback, there can be listener positions where delays from the = speakers=20 interact with the recorded delay to produce comb-filter effects. In = other words,=20 the timbre may change depending on the listener's location. There may be = several=20 randomly located "sweet spot" locations in a room where everything = sounds great=20 and other places where bass is weak and others where it's overbearing = and other=20 places where high end and mid range are problems. Also, when an A-B = mic'd stereo=20 signal is converted to mono - like when heard on most AM radios - = some notes,=20 even whole instruments, can go away because of phase cancellation of = recording=20 articfacts.
There are alternatives to A-B Mic'ing that prevent these problems, MS =
recording for instance. MS is an acronym for Mid-Side, named for the two =
signals=20
that encode the location of sound sources and reverbrant ambiance of the =
stereo=20
image. MS Stereo principles are used in the broadcast delivery of =
Mono/Stereo=20
compatible sound and as part of Surround system but it is how these =
principles=20
apply to microphone selection and placement during recording that is of =
concern=20
here.
The Mid signal comes from a mic with a Cardioid polar sensitivity = oriented=20 so that the "front" faces the center of the stage. This signal is = essentially a=20 mono mix of the Right and Left sides of the stereo field and encodes = very little=20 information on placement of sound sources. For example, if the source = "A" in the=20 illustration is moved from the Right side of the stage to the equivalent = position on the Left, the Mid signal will not change significantly.=20 Mathematically the Mid signal can be expressed as R+L.
The Side signal comes from a mic with a =
Figure 8 polar=20
sensitivity oriented so the lobes are facing Right and Left. Two =
characteristics=20
of this map are important. First, the mic is deaf to sound sources =
located=20
between the two lobes; for example, source "B" in the illustration will =
not be=20
present in the Side signal at all. Also, there is a phase inversion =
between the=20
Right and Left lobes - if source "A" is moved from the Right side of the =
stage=20
to the equivalent postion on the Left, its relative phase will be =
inverted in=20
the output. Math shorthand for this is R-L.
The MS Decoder performs sum and difference operations to =
extract=20
Right and Left level and phase information from the Mid and Side =
signals. In the=20
illustration the circles with "+" signs represent summing functions. The =
triangle represents a 180=BA phase inversion, the equivalent of changing =
the sign=20
of the output relative to the input or multiplying the input by -1.
=
In=20
an actual circuit OpAmps compute sums and phase inversion.
FM Stereo, Stereo TV and even some quad and surround sound systems =
use MS=20
encoded signals because it is an easy path to sending a signal that is=20
mono/stereo compatible. In these systems, the Mid signal modulates the =
main=20
sound carrier while the Side signal modulates a stereo subcarrier. In =
simple=20
monophonic receiving equipment only the main carrier is demodulated and =
the Mid=20
signal is used for a mono output. When stereo outputs are desired, the =
stereo=20
subcarrier is also demodulated and the Side signal combined with the Mid =
signal=20
in a decoder to extract Left and Right Stereo channels.
=20
Order the MS =
Stereo Mic=20
Kit
=20
MS Mic Schematic